use std::io::{Result, Read};
use std::io::{Error, ErrorKind};
use std::net::Ipv4Addr;
use std::fs::File;

pub struct BytePacketBuffer {
    pub buf: [u8; 512],
    pub pos: usize
}

impl BytePacketBuffer {
    pub fn new() -> BytePacketBuffer {
        BytePacketBuffer {
            buf: [0; 512],
            pos: 0
        }
    }

    fn pos(&self) -> usize {
        self.pos
    }

    fn step(&mut self, steps: usize) -> Result<()> {
        self.pos += steps;

        Ok(())
    }

    fn seek(&mut self, pos: usize) -> Result<()> {
        self.pos = pos;

        Ok(())
    }

    fn read(&mut self) -> Result<u8> {
        if self.pos >= 512 {
            return Err(Error::new(ErrorKind::InvalidInput, "End of buffer"));
        }
        let res = self.buf[self.pos];
        self.pos += 1;

        Ok(res)
    }

    fn get(&mut self, pos: usize) -> Result<u8> {
        if pos >= 512 {
            return Err(Error::new(ErrorKind::InvalidInput, "End of buffer"));
        }
        Ok(self.buf[pos])
    }

    fn get_range(&mut self, start: usize, len: usize) -> Result<&[u8]> {
        if start + len >= 512 {
            return Err(Error::new(ErrorKind::InvalidInput, "End of buffer"));
        }
        Ok(&self.buf[start..start+len as usize])
    }

    fn read_u16(&mut self) -> Result<u16>
    {
        let res = ((try!(self.read()) as u16) << 8) |
                  (try!(self.read()) as u16);

        Ok(res)
    }

    fn read_u32(&mut self) -> Result<u32>
    {
        let res = ((try!(self.read()) as u32) << 24) |
                  ((try!(self.read()) as u32) << 16) |
                  ((try!(self.read()) as u32) << 8) |
                  ((try!(self.read()) as u32) << 0);

        Ok(res)
    }

    fn read_qname(&mut self, outstr: &mut String) -> Result<()>
    {
        let mut pos = self.pos();
        let mut jumped = false;

        let mut delim = "";
        loop {
            let len = try!(self.get(pos));

            // A two byte sequence, where the two highest bits of the first byte is
            // set, represents a offset relative to the start of the buffer. We
            // handle this by jumping to the offset, setting a flag to indicate
            // that we shouldn't update the shared buffer position once done.
            if (len & 0xC0) == 0xC0 {

                // When a jump is performed, we only modify the shared buffer
                // position once, and avoid making the change later on.
                if !jumped {
                    try!(self.seek(pos+2));
                }

                let b2 = try!(self.get(pos+1)) as u16;
                let offset = (((len as u16) ^ 0xC0) << 8) | b2;
                pos = offset as usize;
                jumped = true;
                continue;
            }

            pos += 1;

            // Names are terminated by an empty label of length 0
            if len == 0 {
                break;
            }

            outstr.push_str(delim);

            let str_buffer = try!(self.get_range(pos, len as usize));
            outstr.push_str(&String::from_utf8_lossy(str_buffer).to_lowercase());

            delim = ".";

            pos += len as usize;
        }

        if !jumped {
            try!(self.seek(pos));
        }

        Ok(())
    }
}

#[derive(Copy,Clone,Debug,PartialEq,Eq)]
pub enum ResultCode {
    NOERROR = 0,
    FORMERR = 1,
    SERVFAIL = 2,
    NXDOMAIN = 3,
    NOTIMP = 4,
    REFUSED = 5
}

impl ResultCode {
    pub fn from_num(num: u8) -> ResultCode {
        match num {
            1 => ResultCode::FORMERR,
            2 => ResultCode::SERVFAIL,
            3 => ResultCode::NXDOMAIN,
            4 => ResultCode::NOTIMP,
            5 => ResultCode::REFUSED,
            0 | _ => ResultCode::NOERROR
        }
    }
}

#[derive(Clone,Debug)]
pub struct DnsHeader {
    pub id: u16, // 16 bits

    pub recursion_desired: bool, // 1 bit
    pub truncated_message: bool, // 1 bit
    pub authoritative_answer: bool, // 1 bit
    pub opcode: u8, // 4 bits
    pub response: bool, // 1 bit

    pub rescode: ResultCode, // 4 bits
    pub checking_disabled: bool, // 1 bit
    pub authed_data: bool, // 1 bit
    pub z: bool, // 1 bit
    pub recursion_available: bool, // 1 bit

    pub questions: u16, // 16 bits
    pub answers: u16, // 16 bits
    pub authoritative_entries: u16, // 16 bits
    pub resource_entries: u16 // 16 bits
}

impl DnsHeader {
    pub fn new() -> DnsHeader {
        DnsHeader { id: 0,

                    recursion_desired: false,
                    truncated_message: false,
                    authoritative_answer: false,
                    opcode: 0,
                    response: false,

                    rescode: ResultCode::NOERROR,
                    checking_disabled: false,
                    authed_data: false,
                    z: false,
                    recursion_available: false,

                    questions: 0,
                    answers: 0,
                    authoritative_entries: 0,
                    resource_entries: 0 }
    }

    pub fn read(&mut self, buffer: &mut BytePacketBuffer) -> Result<()> {
        self.id = try!(buffer.read_u16());

        let flags = try!(buffer.read_u16());
        let a = (flags >> 8) as u8;
        let b = (flags & 0xFF) as u8;
        self.recursion_desired = (a & (1 << 0)) > 0;
        self.truncated_message = (a & (1 << 1)) > 0;
        self.authoritative_answer = (a & (1 << 2)) > 0;
        self.opcode = (a >> 3) & 0x0F;
        self.response = (a & (1 << 7)) > 0;

        self.rescode = ResultCode::from_num(b & 0x0F);
        self.checking_disabled = (b & (1 << 4)) > 0;
        self.authed_data = (b & (1 << 5)) > 0;
        self.z = (b & (1 << 6)) > 0;
        self.recursion_available = (b & (1 << 7)) > 0;

        self.questions = try!(buffer.read_u16());
        self.answers = try!(buffer.read_u16());
        self.authoritative_entries = try!(buffer.read_u16());
        self.resource_entries = try!(buffer.read_u16());

        // Return the constant header size
        Ok(())
    }
}

#[derive(PartialEq,Eq,Debug,Clone,Hash,Copy)]
pub enum QueryType {
    UNKNOWN(u16),
    A, // 1
}

impl QueryType {
    pub fn to_num(&self) -> u16 {
        match *self {
            QueryType::UNKNOWN(x) => x,
            QueryType::A => 1,
        }
    }

    pub fn from_num(num: u16) -> QueryType {
        match num {
            1 => QueryType::A,
            _ => QueryType::UNKNOWN(num)
        }
    }
}

#[derive(Debug,Clone,PartialEq,Eq)]
pub struct DnsQuestion {
    pub name: String,
    pub qtype: QueryType
}

impl DnsQuestion {
    pub fn new(name: String, qtype: QueryType) -> DnsQuestion {
        DnsQuestion {
            name: name,
            qtype: qtype
        }
    }

    pub fn read(&mut self, buffer: &mut BytePacketBuffer) -> Result<()> {
        try!(buffer.read_qname(&mut self.name));
        self.qtype = QueryType::from_num(try!(buffer.read_u16())); // qtype
        let _ = try!(buffer.read_u16()); // class

        Ok(())
    }
}

#[derive(Debug,Clone,PartialEq,Eq,Hash,PartialOrd,Ord)]
#[allow(dead_code)]
pub enum DnsRecord {
    UNKNOWN {
        domain: String,
        qtype: u16,
        data_len: u16,
        ttl: u32
    }, // 0
    A {
        domain: String,
        addr: Ipv4Addr,
        ttl: u32
    }, // 1
}

impl DnsRecord {

    pub fn read(buffer: &mut BytePacketBuffer) -> Result<DnsRecord> {
        let mut domain = String::new();
        try!(buffer.read_qname(&mut domain));

        let qtype_num = try!(buffer.read_u16());
        let qtype = QueryType::from_num(qtype_num);
        let _ = try!(buffer.read_u16());
        let ttl = try!(buffer.read_u32());
        let data_len = try!(buffer.read_u16());

        match qtype {
            QueryType::A  => {
                let raw_addr = try!(buffer.read_u32());
                let addr = Ipv4Addr::new(((raw_addr >> 24) & 0xFF) as u8,
                                         ((raw_addr >> 16) & 0xFF) as u8,
                                         ((raw_addr >> 8) & 0xFF) as u8,
                                         ((raw_addr >> 0) & 0xFF) as u8);

                Ok(DnsRecord::A {
                    domain: domain,
                    addr: addr,
                    ttl: ttl
                })
            },
            QueryType::UNKNOWN(_) => {
                try!(buffer.step(data_len as usize));

                Ok(DnsRecord::UNKNOWN {
                    domain: domain,
                    qtype: qtype_num,
                    data_len: data_len,
                    ttl: ttl
                })
            }
        }
    }
}

#[derive(Clone, Debug)]
pub struct DnsPacket {
    pub header: DnsHeader,
    pub questions: Vec<DnsQuestion>,
    pub answers: Vec<DnsRecord>,
    pub authorities: Vec<DnsRecord>,
    pub resources: Vec<DnsRecord>
}

impl DnsPacket {
    pub fn new() -> DnsPacket {
        DnsPacket {
            header: DnsHeader::new(),
            questions: Vec::new(),
            answers: Vec::new(),
            authorities: Vec::new(),
            resources: Vec::new()
        }
    }

    pub fn from_buffer(buffer: &mut BytePacketBuffer) -> Result<DnsPacket> {
        let mut result = DnsPacket::new();
        try!(result.header.read(buffer));

        for _ in 0..result.header.questions {
            let mut question = DnsQuestion::new("".to_string(),
                                                QueryType::UNKNOWN(0));
            try!(question.read(buffer));
            result.questions.push(question);
        }

        for _ in 0..result.header.answers {
            let rec = try!(DnsRecord::read(buffer));
            result.answers.push(rec);
        }
        for _ in 0..result.header.authoritative_entries {
            let rec = try!(DnsRecord::read(buffer));
            result.authorities.push(rec);
        }
        for _ in 0..result.header.resource_entries {
            let rec = try!(DnsRecord::read(buffer));
            result.resources.push(rec);
        }

        Ok(result)
    }
}

fn main() {
    let mut f = File::open("response_packet.txt").unwrap();
    let mut buffer = BytePacketBuffer::new();
    f.read(&mut buffer.buf).unwrap();

    let packet = DnsPacket::from_buffer(&mut buffer).unwrap();
    println!("{:?}", packet.header);

    for q in packet.questions {
        println!("{:?}", q);
    }
    for rec in packet.answers {
        println!("{:?}", rec);
    }
    for rec in packet.authorities {
        println!("{:?}", rec);
    }
    for rec in packet.resources {
        println!("{:?}", rec);
    }
}